Sky Brightness

The maximum acceptable sky brightness for an observation is defined as a single constraint expressed in terms of the number of magnitudes the r-band sky is brighter then best possible dark sky conditions. The sky brightness model used, courtesy of Ivan Baldry, was derived from several years of real LT RATCam data and incorporates solar elevation, lunar phase, lunar elevation, target elevation and distance of target from the Moon. For example, the same moderate sky brightness arises from a gibbous Moon low on the horizon or a crescent Moon high in the sky. (Many other examples showing the influence of Sun and Moon on the total sky brightness are shown in the pictograms below.)

Variation with wavelength or filter

As with seeing, the scheduling constraint is based on the r'-band irrespective of what filters or instrument you use. Comparing to other filter passbands:

Vr'i' are all very similar to each other

u'Bg' are about one mangitude more strongly effected by twilight and moonlight

z' band is about one magnitude less effected.

Note again that
the criterion is expressed relative to the best possible dark sky in that filter. We are not saying the absolute
sky flux in Vr'i' are all the same, only that in astronomical twilight, each is about two magnitudes brighter than
it would be in full dark.

Groups without a Sky Brightness Constraint

As with all Observing Constraints, a group with no sky brightness set is
completely unconstrained. I.e., you are happy to observe from the moment the Sun is below the horizon;
perfectly legal, but not likely what you want.

Availability of the different brightness classes

The precise amount of time available to observe any given target on any given night is dependent
on both the target's and Moon's positions (RA,dec). It varies through the month as the Moon waxes, wanes
and indeed moves. For high declination fields the Moon may never come particularly close to your target.
The following plots have been derived by averaging simulations of three
years of observing conditions for targets at a range of declinations.

Scheduling is performed in terms of "observe whenever the sky is equal to or darker than the specified limit"
and not "observe when the sky brightness is equal to the specified class" so cumulative plots are most
important for deciding how likely your target is to be observed.
Though not as relevant to the scheduler, plots of the differential distributions
are also included below.

Total cumulative availabilty of observing conditions averaged over a long time
period. Roughly 40% of the time the sky is 'dark' whereas about 65% of the time
it is equal to or darker than '2mag' which is what would traditionally have been
described as 'astronomical twilight'.

Plots illustrating how the availability of different conditions varies through
a lunation. For a particular source in a particular month this plot can vary
hugely. These plots represent an 'average month' derived from three years' worth
of simulations. The greatest difference between the two panels is that for a high
declination source, the Moon never gets close enough to push the sky into the
'+6mag' class so the magenta '+4mag' line is determined entirely by twilight, not
by Moon phase.

Plots illustrating how the availability of different conditions varies through
a lunation, here plotting the amount of time the telescope would be in each class, not
"in or darker than" the specified class. This plot is thus not directly relevent to
the scheduler, but does make it easier to visualize how the sky brightness varies
through the month.

A Worked Example

Take a hypothetical group that requests observations once every night but with no
constraint on what time of night (i.e., a simple MONITOR with one-day period)
and a sky brightness constraint of 2 mag.

The red and yellow bar chart "Cumulative Availability of Sky Brightness" says
that averaged over a year, 65% of all time is "2mag or better". This plot however
says nothing about individual nights. As far as this plot shows there might be some
amount of 2mag time every night and the specified group might in fact be done every night.
Obviously some nights it would have more chance than others.

The the next pair of plots "Availability of sky brightness through a lunation" does show
something about individual nights. This shows the availability on each night through a
month as the Moon moves. The cyan curve is for "2mag or darker". You see that at full
Moon there is essentially no available time and really there is hardly any "2mag"
available about 2 - 3 nights either side of full Moon.

Putting that together, about 65% of all time in the year is darker than 2mag,
but there are 4 - 5 nights each month when there is very little and such a great is
unlikely to be observable.

Remember also that these plots are averaged for some arbitrary "typical" target Ra,dec.
A target in the ecliptic will certainly have a few nights each lunation when it is
completely impossible whereas a target far out of the ecliptic might have a small amount
of time when it is possible every night. You see this difference between the equatorial
and high-latitude plots above. The equatorial one is much more strongly effected by
the Moon in the 4mag (magenta) class.

Descriptive pictograms

The Phase2UI allows you to explicitly choose from a list of various sky
brightnesses in magnitudes The following pictograms illustrate what brightness setting
would arise from various familiar observing conditions.

Sun
Altitude > -4 degrees

and

Moon
Any altitude, any phase.

Sun
Altitude < -4 degrees

and

Moon
Any altitude, any phase.

Effectively "Nautical Twilight"

Sun
Altitude < -8 degrees

and

Moon
Any altitude, any phase, minimum distance 10°

Effectively "Astronomical Twilight" or "full moon"

Sun
Altitude < -12 degrees

and

Moon
Full moon at any altitude, minimum distance 30°

or

Minimum distance 15°, at:
any altitude & up to 65% illuminated or
up to 60° alt & 75% phase or
up to 8° alt & full phase

Effectively "Astronomical Twilight" or "high gibbous moon"

Sun
Altitude < -12 degrees

and

Moon

Minimum distance 30°, at:
any altitude & up to 65% illuminated, or
up to 60° alt & 75% illuminated, or
up to 8° alt & full phase

or

Minimum distance 15°, at:
any altitude & up to 45% illuminated, or
up to 45° alt & half phase, or
up to 8° & 75% phase